API output without Persian diacritics or written ezafe ی (company decision); tutor answers too

plain_persian() strips kasra/fatha/damma/shadda/sukun/hamza-above and the written «ه‌ی» ezafe, keeping tanwin. Applied
at output, not in data: PlainPersianJSONRenderer runs one byte-level pass over every JSON response (~5 ms per MB
locally), and to_plain() cleans tutor answers including the stream. Rewriting titles in the DB/seed would have broken
question binding by title path and SeedRecord keys. Tests updated to expect the cleaned text; config/tests_plain_persian.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ai_tutor/tests_concurrency.py

@@ -50,7 +50,7 @@ class AskConcurrencyCapTests(TestCase):
         finally:
             concurrency.release()
         self.assertEqual(response.status_code, 503)
-        self.assertEqual(response.json()['detail'], 'دستیار الان خیلی شلوغه؛ چند ثانیه‌ی دیگه دوباره بپرس.')
+        self.assertEqual(response.json()['detail'], 'دستیار الان خیلی شلوغه؛ چند ثانیه دیگه دوباره بپرس.')
         self.assertEqual(response['Retry-After'], '10')
         self.assertFalse(Message.objects.exists())  # نه پیامی ساخته شد نه سهمیه‌ای رزرو شد